The invention relates to sensor systems and, mcre particularly, to scanning sensor systems.
Various types of sensor systems are known which can passively detect radiation, such as infrared radiation, emitted by objects within a field of view, analyze the position of such objects, and provide an output of information regarding the characteristics and movement of such objects. A first type of sensor system is known as a "staring" sensor system, in that the entire field of view is focused upon an array of infrared detector elements arranged in a matrix. The field of view remains focused in a stationary manner upon the detector array and the outputs from the various detectors are read out and processed. A staring sensor system provides maximum opportunity for detector elements to integrate the energy of infrared radiation impinging thereon, resulting in good sensitivity. However, the spatial sampling rate of staring type sensor systems is determined by the physical size of each detector element. Moreover, staring sensor systems typically handle only small fields of view on the order of two degrees by two degrees.
Scanning systems avoid some of the disadvantages of staring type systems. In such systems, radiation from a portion of the field of view is scanned across an array of detector elements, and the output signals of the detector elements are sampled and multiplexed for application to processing circuitry. The rate at which the full field of view can be processed is called the "frame rate." Spatial sample rate in scanning systems can be controlled by the detector signal sample rate and sensor scan rate. However, scanning sensor systems are also subject to disadvantages. In order to provide a rapid coverage of a desired field of view, it is necessary to scan the image across the detector element array at high speed. However, this reduces the dwell time of the image upon each individual detector element, thereby correspondingly reducing the amount of energy which can be integrated by each detector element to provide an output signal detectable above the noise level. Moreover, a high sampling rate is required to obtain the desired spatial resolution.
A reduction in sampling rate can be provided by slowing the scan rate of the system. However, sensor systems are typically employed where it is desired to monitor a field of view, detect a target, and return to the same position in the field of view to determine if the target has moved. It is additionally desirable to provide a high frame rate, that is, to rapidly return to an original field of view portion in order to detect variations, or modulations, in intensity of radiation emitted by the target, thus obtaining valuable information concerning the characteristics of the target. If the scan rate of scanning type sensors is slowed to permit longer integration times and slower sampling rates, this reduces the frame rate of the system, resulting in a corresponding reduction in the ability to frame-to-frame associate closely spaced objects and measure modulation of targets by comparing target characteristics on successive frames.
Various methods have been employed to increase the performance of scanning systems, one of which is known as time delay integration (TDI). In this technique, output signals produced by each detector element are sampled as the array is scanned, with the sampled output signals of adjacent detector elements being provided to time delay and summation circuitry. The signals produced by multiple detector elements are thus superimposed in time and summed to provide an increased signal to noise ratio compared to that of a single detector.
It is now possible to employ hundreds and even thousands of individual detector elements upon a focal plane array to provide increased performance using a scanning TDI system. However, such systems still require rapid sampling, which in turn produces an extremely high data output rate. Performance increases from large numbers of individual detector elements are thus limited by the complexity of processing circuitry required by such high data rates and by limitations on the number of interconnections which must be provided from the focal plane array to the off-array processing circuitry.
Another type of sensor system which has been proposed is known as a "step/stare" system. In this system, target object radiation collected by system optics is held stationary across an array or portion of an array of detector elements to permit the detector elements to integrate the received radiation. The entire matrix is then stepped to a different portion of the field of view and permitted to "stare" at, or integrate in a stationary manner, the radiation received from the new viewed portion of the field of view. A problem encountered in step/stare systems is that individual detector elements have non-identical response characteristics. That is, the threshold, gain, and frequency response of individual detector elements may vary. This variation causes undesired fluctuations in output signals known as pattern noise which is especially troublesome in step/stare systems. This and other problems, such as limited two-dimensional spatial sample rates, have combined to reduce the popularity of step/stare systems.
It is therefore an object of the present invention to provide a non-staring sensor system having a reduced sampling rate.
It is a further object of the present invention to provide a sensor system which reduces the interconnections between the focal plane array and associated processing circuitry.
It is yet another object of the present invention to provide a sensor system in which integration times for individual detector elements are increased while maintaining a high effective frame rate.